Exhaust gases are generated in many industrial and transportation applications. Environmental concerns as well as industrial consequences of release of pollutants or contaminants require their elimination or reduction. In recent times there has been a greater emphasis on the reduction of pollutants emitted in smoke plumes, whether of factories, electricity generating stations, vehicles or ships. Similarly there has also been an emphasis on the removal, or conversion, of toxic chemicals emitted from industrial processes, whether in the pulp and paper, plastics, or other industries. There has also been a desire to reduce the heat emitted by engine exhaust systems, whether for the purpose of achieving greater economies by trapping and reusing waste heat for secondary and tertiary activities or for reducing the infra-red heat signature of an engine intended for military use. Further, a scrubber may, as one of its features, not only remove undesired elements, but may also reduce the noise of an exhaust flow.
There are many examples of specific instances when scrubbing is desirable. For example it may be desired to remove gaseous and fine particulate matter contaminants, odorous compounds and other undesirable elements from exhaust gases emanating from combustion of fossil fuels, whether gas, fuel oil, diesel oil and other petroleum products. The fuels are commonly used in marine diesel engines and boilers, as well as diesel engines used for transportation and construction equipment. Sulfur dioxide is a particular component of many processes involving combustion, ranging from thermoelectric generation, waste incineration, industrial processes, and exhaust gases of prime movers, including diesel engines. In some instances such as with forestry or mining equipment, use of a water scrubbing medium is also desired to discourage or eliminate spark emission.
In another field, it is desirable to scrub exhaust gases emanating from industrial processes such as chemical processes, heat transfer processes, food preparation, agricultural operations, mechanical parts cleaning, paint spray operations and similar processes. Similarly, it may be desired to treat products of the combustion of solid, liquid and gaseous fuels such as biomass, coal, coal water slurry, coal and limestone water slurry, coal methanol slurry. Further still, scrubbing may be required for products of combustion from incineration systems for the thermal destruction of solid, liquid or gaseous waste products. These can include industrial and municipal wastes, biomedical wastes, hazardous and pathological solid and liquid wastes, and solids and liquids contaminated with toxic, hazardous, and pathological wastes, accidental hazardous and dangerous waste spills, and similar waste products. Further still gas/water interaction may be used to clean and humidify exhaust gas for re-introduction of moist gas to the air intake of a combustion appliance for temperature control such as may be seen in a nitrogen oxide reduction scheme.
Scrubbers of various types are known. Removal of fine particles of dust, oxides of sulfur and nitrogen, odorous compounds, and similar contaminants from gas streams is a priority for environmental control abatement programs developed by regulatory agencies to minimize the impact of industrial processes on the natural environment by reducing acidification, ozone formation, nutrient generation and related adverse effects. Devices currently in use for removal of pollutants include cyclones, bag filters, electrostatic precipitators, and high energy scrubbers. Typically the input to output efficiency of these devices range from 85% to ≧99.99%, with the high energy scrubbers being the most efficient, and the cyclone and inertial separators the least. Input to output efficiency is defined as the total concentration of particles of all size ranges in the outlet gas stream from the system as a percentage of the concentration in the total input to the gas cleaning unit. In many combustion appliances, such as a diesel engine for example, the use of exhaust scrubbers is restricted due to the low available back pressure, necessitating a large, costly and unreliable exhaust gas blower.
The type of unit for a specific application is determined by a number of factors including type of industrial process, type and size of particle released, temperature of the gas stream, process economics, land use adjacent to the site, and a number of other factors. High energy scrubbers using limestone and water slurry scrubbing solutions have been successfully used to scrub sulphur from the combustion gases produced when burning sulfur containing fuels, such as coal, heavy fuel oil, and so on.
A common method of scrubbing, for example, exhaust gases, is to spray a scrubbing medium, such as water, across the exhaust gas passage, or to force the exhaust gases through a continuously fed curtain of water, or along a channel with wetted sides. These technologies for scrubbing fine particles from gaseous streams have relied on mechanical shear systems to produce large quantities of fine droplets of scrubbing solution. In each instance droplet surface area is the controlling parameter determining the efficiency of the scrubber. To increase scrubber droplet surface area for a given water mass, the average droplet diameter must decrease. The energy required to decrease the average droplet size and thus increase the average droplet surface area increases sharply. Thus the efficiency of conventional scrubbers for fine particle removal is a function of the energy input as measured by the pressure loss across the scrubber. Typical high efficiency scrubbers (>99% efficiency) operate with pressure drops in the range of 45–60 inches of water. Such units have high capital costs, and high energy and maintenance costs.